This invention relates to a method of recording and reproducing a line sequential color video signal, and more particularly to the elimination of color errors in a reproducing system.
In countries including France, a line sequential television system called the SECAM system is a standard television system. Two color difference signals R-Y and B-Y are alternately switched over to be selected at a time interval of one horizontal scanning period (abbreviated here in after as a period of 1H) and are used for frequency modulation of a subcarrier, and a luminance signal is superposed on the frequency-modulated color difference signals to constitute a carrier color video signal. In a reproducing system reproducing the carrier color video signal, the color difference signal delayed by the period of 1H and the color difference signal not subjected to the delay are derived in parallel relation, so that the color difference signals R-Y and B-Y dropped out at the time interval of the period of 1H at the time of recording are complemented by the color difference signals R-Y and B-Y demodulated in the preceding horizontal scanning periods to obtain the two independent and continuous color difference signals R-Y and B-Y. For this purpose, the reproducing system includes a switch for alternately deriving the color difference signal delayed by the period of 1H and the color difference signal not subjected to the delay. Deriving the two kinds of color difference signals in parallel relation as described above is called the simultaneous, and the switch provided for that purpose is called a simultaneous switch.
FIGS. 1(a) to 1(e) illustrate conceptionally the structure of various color difference signals appearing in a system resorting to the line sequential method. In FIGS. 1(a) to 1(e), suffixes attached 1, 2, 3, 4, --- to the symbol Y indicate the numbers of the horizontal scanning lines to which specific color difference signals R-Y and B-Y belong, and each block corresponds to the period of 1H. Color difference signals R-Y and B-Y shown in FIGS. 1(a) and 1(b) respectively are alternately selected by a line sequential switch at the time interval of the period of 1H to provide line sequential color difference signals (R-Y)1 and (B-Y)1 shown in FIG. 1(c). In this case, the line sequential color difference signals (R-Y)1 and (B-Y)1 include the color difference signals R-Y and B-Y belonging to the odd-numbered and even-numbered periods of 1H respectively as shown. In the reproducing, system, such line sequential color difference signals (R-Y)1 and (B-Y)1 are delayed and not delayed by the period of 1H, and the delayed signals and the thru signals are applied to the simultaneous switch to provide two kinds of simultaneous color difference signals (R-Y)2 and (B-Y)2 as shown in FIG. 1(d) and 1(e) respectively.
It will be apparent from reference to FIGS. 1(d) and 1(e) that, according to the line sequential method, the line sequential color difference signal (R-Y)1 delayed by the period of 1H provides the signal belonging to the even-numbered 1H period of the simultaneous color difference signal (R-Y)2. And the line sequential color difference signal (B-Y)1, delayed by the period of 1H provides the signal belonging to the odd-numbered 1H period of the simultaneous color difference signal (B-Y)2. And color problem does not arise when there is a correlation between the color difference signal R-Y belonging to a certain period of 1H and the color difference signal B-Y belonging to the next adjacent period of 1H. However, when there is a great variation between the color difference signal R-Y belonging to a certain period of 1H and the color difference signal B-Y belonging to the next adjacent period of 1H, a color deviating greatly from the original color will be reproduced. Suppose, for example, that there is a great variation between the color difference signals R-Y3 and B-Y3 belonging to the 3rd period of 1H and the color difference signals R-Y4 and B-Y4 belonging to the 4th period of 1H. Since, in such a case, the color difference signals R-Y3 and B-Y4 in the respective simultaneous color difference signals (R-Y4)2 and (B-Y4)2 are those corresponding to the 4th period of 1H, reproduction of the color difference signal R-Y3 will give rise to a color error.
FIG. 2 is a block diagram showing the structure of a prior art reproducing system which is designed to reduce the color error described above. Referring to FIG. 2, a simultaneous switch 1 includes a first switch 2 generating the simultaneous color difference signal (R-Y)2 and a second switch 3 generating the simultaneous color difference signal (B-Y)2. The first switch 2 has contacts 2a and 2b, and the second switch 3 has contacts 3a and 3b. A control Pulse signal Pc is applied directly to the first switch 2 and through an inverter 4 to the second switch 3 for changing over the switches 2 and 3 at a time interval of the period of 1H, so that the switch contacts 2a, 3b and the switch contacts 2b, 3a can be simultaneously selected respectively. The line sequential color difference signals (R-Y)1 and (B-Y)1 delayed by the period of 2H by two 1H delay circuits 5 and 6 are applied to a mixer 7 together with the line sequential color difference signals (R-Y)1 and (B-Y)1 not subjected to the delay. The color difference signals (R-Y)1 and (B-Y)1 applied to the mixer 7 are mixed together, and the resultant signal is divided by the factor of 2 in the mixer 7 to appear as a color difference output signal
(thru)+(2HDL)/2 which is applied to the switch contacts 2a and 3a. On the other hand, the line sequential color difference signals (R-Y)1 and (B-Y)1 delayed by the period of 1H by the 1H delay circuit 5 are applied to the switch contacts 2b and 3b.
FIGS. 3(a) to 3(d) illustrate conceptionally the structure of the simultaneous color difference signals (R-Y)2 and (B-Y)2 produced in the reproducing system shown in FIG. 2 and illustrate also the relation between these signals and the color difference signals R-Y and B-Y shown in FIG. 1 on the same time base.
It will be apparent from reference to FIG. 3(a) to 3(d) that the simultaneous color difference signal (R-Y)2 is such that the color difference signal (R-Yn)+(R-Yn-2)/2, which is provided by dividing, by the factor of 2, the sum of the color difference signal (R-Yn-2) applied at the time preceding the present time by the period of 2H and the color difference signal (R-Yn) applied at the present time, is followed by the color difference signal (R-Yn), replacing the color difference signal (R-Yn+1) applied at the time preceding the present time by the period of 1H, and such a relation is sequentially repeated. Also, the simultaneous color difference signal (B-Y)2 is such that the color difference signal (B-Yn-1) applied at the time preceding the present time by the period of 1H is followed by the color difference signal (B-Yn+1)+(B-Yn-1)/2 provided by dividing, by the factor of 2, the sum of the color difference signal (B-Yn-1) applied at the time preceding the present time by the period of 2H and the color difference signal (B-Yn+1) applied at the present time, and such a relation is sequentially repeated.
Therefore, the adverse effect attributable to the variation of the color difference signals R-Y and B-Y can be reduced to the half, but the adverse effect appears still over the period of 2H.
When the circuit shown in FIG. 2 is employed, the luminance signal Y is also delayed by the period of 1H although not illustrated.
The simultaneous color difference signals obtained by the methods shown in FIGS. 1 and 3 were directly converted into NTSC standard video signals through an encoder, and these video signals were actually reproduced on a screen of a television receiver for the purpose of comparison. As a result, it has been found that although the absolute value of the color error is large in the case of the method shown in FIG. 1, the color error is not so marked when compared with that observed in the case of the method shown in FIG. 3 in which the color error appears over the period of 2H.
A common practice for converting the simultaneous color difference signals (R-Y)2 and (B-Y)2 into an NTSC standard video signal includes subjecting two kinds of carriers to balanced modulation by the simultaneous color difference signals (R-Y)2 and (B-Y)2 respectively, mixing the thus obtained, simultaneous carrier color difference signals (R-Y)3 and (B-Y)3, and then mixing the resultant signal mixture with the luminance signal Y. In this connection, in an attempt to improve the S/N ratio of the simultaneous carrier color difference signals (R-Y)3 and (B-Y)3, the inventor of the present invention has thought of applying the simultaneous carrier color difference signals (R-Y)3 and (B-Y)3 to a comb filter.
Describing in more detail, this comb filter 10 is composed of a 1H delay circuit 10a and a mixer as 10b, as shown in FIGS. 2 and 4. The thru simultaneous carrier color difference signals (R-Y)3 and (B-Y)3 are applied, together with the delayed simultaneous carrier color difference signals (R-Y)3 and (B-Y)3 delayed by the period of 1H by the 1H delay circuit 10a, to the mixer 10b which generates output signals (R-Y)4 and (B-Y)4 divided by the factor of 2. Such an arrangement is advantageous in that, while the carrier color difference signals (R-Y)3 and (B-Y)3 are added together, the level of noise occurring at random does not become as high as that of the carrier color difference signals (R-Y)3 and (B-Y)3, so that the S/N ratio can be improved correspondingly. In FIG. 4 reference numerals 37, 38 and 42 designate a 1H delay circuit, a simultaneous, switch and a balanced modulator, respectively.
FIG. 3(e) and 3(f) illustrate conceptionally the structure of the simultaneous carrier color difference signals (R-Y)4 and (B-Y)4 provided when the simultaneous carrier color difference signals (R-Y)3 and (B-Y)3, obtained by the prior art circuit shown in FIG. 2, are passed through the comb filter 10 so as to improve the S/N ratio. It will be apparent from FIG. 3(e) that, in the simultaneous carrier color difference signal (R-Y)4 generated from the comb filter 10, one of the color difference signals, for example, the color difference signal (R-Y1) appears over the consecutive period of 4H, and the succeeding color difference signal (R-Y2) appears later by the period of 2H than the color difference signal (R-Y)1 and lasts over the period of 4H, due to the provision of the 1H delay circuits 5, 6 and 10b. It will be similarly apparent from FIG. 3(f) that, in the other simultaneous carrier color difference signal (B-Y)4 generated from the comb filter 10, one of the color difference signals, for example, the color difference signal (B-Y1) appears over the period of 4H, and the succeeding color difference signal (B-Y2) appears later by the period of 2H than the color difference signal (B-Y1) and lasts over the period of 4H. The signals (R-Y)4 and (B-Y)4 described above are periodically generated with a delay time of 1H relative to each other. Therefore, when the prior art circuit having the structure shown in FIG. 2 is used, the adverse effect attributable to variations of the signals R-Y and B-Y lasts over the period of 4H.
FIGS. 5(a) to 5(e) illustrate conceptionally the structure of the color difference signals appearing from the various parts of the circuit shown in FIG. 4 and are similar to FIGS. 1(a) to 1(e) respectively. It will be seen in FIGS. 5(a) to 5(c) that the line sequential color difference signals (R-Y)1, (B-Y)1, the simultaneous color difference signals (R-Y)2, (B-Y)2 and the simultaneous carrier color difference signals (R-Y)3, (B-Y)3 have structures similar to those shown in FIGS. 1(a) to 1(e).
On the other hand, the simultaneous carrier color difference signal (R-Y)4 generated from the comb filter 10 has a structure as shown in FIG. 5(d). It will be seen in FIG. 5(d) that, in the case of an odd-numbered or (2n+1)th horizontal scanning line, the output of the comb filter 10 is the signal generally expressed as (R-Y2n-1)+(R-Y2n+1)/2, that is, the signal provided by dividing, by the factor of 2, the sum of the color difference signal (R-Y2n+1) belonging to the (2n+1)th horizontal scanning line and the color difference signal (R-Y2n-1) belonging to the horizontal scanning line preceding the (2n+1)th horizontal scanning line by the period of 2H. On the other hand, in the case of an even-numbered or 2(n+1)th horizontal scanning line, the output of the comb filter 10 is the signal generally expressed as (R-Y2n-1), that is, the signal provided by dividing, by the factor of 2, the sum of the color difference signals (R-Y2n-1 ) belonging to the horizontal scanning line preceding the 2(n+1)th horizontal scanning line by the period of 1H. Similarly, the simultaneous carrier color difference signal (B-Y)4 generated from the comb filter 10 has a structure as shown in FIG. 5(e). It will be seen in FIG. 5(e) that, in the case of an odd-numbered or (2n+1)th horizontal scanning line, the output of the comb filter 10 is the signal generally expressed as (B-Y2n), that is, the signal provided by dividing, by the factor of 2, the sum of the color difference signals (B-Y2n) belonging to the horizontal scanning line preceding the (2n+1)th horizontal scanning line by the period of 1H. On the other hand, in the case of an even-numbered or 2(n+1)th horizontal scanning line, the output of the comb filter 10 is generally expressed as (B-Y2n)+(B-Y2(n+1)/2, that is, the signal provided by dividing, by the factor of 2, the sum of the color difference signal (B-Y2(n+1) belonging to the 2(n+1)th horizontal scanning line and the color difference signal (B-Y2n) belonging to the horizontal scanning line preceding the 2(n+1)th horizontal scanning line by the period of 2H.
Direct mixing of the luminance signal Y as shown in FIG. 5(f) with those carrier color difference signals (R-Y)4 and (B-Y)4 may give rise to a large color error resulting in a degraded quality of the reproduced picture, when great variations occur in the color difference signals R-Y and B-Y between a certain horizontal scanning line and the next adjacent horizontal scanning line.